By most medical standards, the skin is considered an organ of the human body and carries out many vital functions such as thermal regulation and protection against external biological microorganisms. The dermis and epidermis vary in thickness greatly depending on location on the body. The epidermis varies from roughly 50-micron thick on “thin-skin” areas such as the eyelids, to over 1-mm thick on heavy-use areas such as the palms of the hands or soles of the feet. The average value around the entire body is roughly 100 micron. And the epidermis is a stratified squamous epithelial tissue, meaning that does not have its own blood supply. Instead, it relies solely on the blood supply from the dermis, which is its primary source of water and nourishment.
Independent of location on the body, the skin displays a characteristic variation of hydration with depth as illustrated in the hydration curve 100 in FIG. 1. Hydration is defined as the fraction of water in the skin tissue per unit mass, following the convention established for all “soft” tissue. More specifically, it is water in the aqueous state, not the “bound” state whereby water is chemically bonded to biomolecules (e.g., proteins) or other biological structures. The outermost sublayer of the epidermis, the stratum corneum, typically has about 20% hydration. This abruptly increases to approximately 60% in the innermost sublayer, the basale corneum. Once into the dermis, the hydration levels off to approximately 65% in a typical, healthy person, which is roughly the hydration level in soft tissue of all sorts.
Significant variations, however, can occur because of skin maladies or because of underlying disease. For example, skin burns introduce a lateral variation in hydration depending on the severity of the burn, be it 1st degree (which damages only the epidermis), 2nd degree (which extends into the dermis, either partially or fully), and 3rd degree (damage into the subcutaneous). Carcinomas generally involve only the epidermis, and melanomas usually start in the epidermis but then grow vertically into the dermis where they can readily metastasize through the blood supply there (which is why melanomas are more deadly than carcinomas). Skin hydration can also be affected by internal disease, which affects the skin tissue through edema—the abnormality associated with leaking of blood vessels into surrounding soft tissue, which usually raises the fluid and hydration levels and leads to overall swelling. A common internal disease that causes this is congestive heart failure. Another is clot- or tumor-blockage of major blood vessels.